1. Field of the Invention
The present invention relates to computer networks and, more specifically, to computer networks that have been segmented into a plurality of Virtual Local Area Net-works (VLANs).
2. Background Information
A computer network typically comprises a plurality of interconnected entities. An entity may consist of any network device, such as a server or end station, that “sources” (i.e., transmits) or “sinks” (i.e., receives) data frames. A common type of computer network is a local area network (“LAN”) which typically refers to a privately owned network within a single building or campus. LANs typically employ a data communication protocol (LAN standard), such as Ethernet, FDDI or token ring, that defines the functions performed by the data link and physical layers of a communications architecture (i.e., a protocol stack).
One or more intermediate network devices are often used to couple LANs together and allow the corresponding entities to exchange information. For example, a bridge may be used to provide a “bridging” function between two or more LANs or end stations. Typically, the bridge is a computer and includes a plurality of ports that are coupled to the LANs or end stations. Ports used to couple bridges to each other are generally referred to as a trunk ports, whereas ports used to couple bridges to LANs or end stations are generally referred to as access ports. The bridging function includes receiving data from a sending entity at a source port and transferring that data to at least one destination port for forwarding to a receiving entity.
Although bridges may operate at various levels of the communication protocol stack, they typically operate at Layer 2 (L2) which, in the OSI Reference Model, is called the data link layer and includes the Logical Link Control (LLC) and Media Access Control (MAC) sub-layers. Data frames at the data link layer typically include a header containing the MAC address of the entity sourcing the message, referred to as the source address, and the MAC address of the entity to whom the message is being sent, referred to as the destination address. To perform the bridging function, L2 bridges examine the MAC destination address of each data frame received on a source port. The frame is then switched onto the destination port(s) associated with that MAC destination address.
Other devices, commonly referred to as routers, may operate at higher communication layers, such as Layer 3 (L3) of the OSI Reference Model, which in Transmission Control Protocol/Internet Protocol (TCP/IP) networks corresponds to the Internet Protocol (IP) layer. Packets at the IP layer also include a header which contains an IP source address and an IP destination address. Routers or L3 switches may re-assemble or convert received data frames from LAN standard (e.g., Ethernet) to another (e.g. token ring). Thus, L3 devices are often used to interconnect dissimilar subnetworks.
Virtual Local Area Networks
A computer network, such as LAN, may also be segregated into a series of logical network segments. For example, U.S. Pat. No. 5,394,402, issued on Feb. 28, 1995 to Ross (the “'402 Patent”), which is hereby incorporated by reference in its entirety, discloses an arrangement for associating any port of a bridge with any particular segregated network group. Specifically, according to the '402 Patent, any number of physical ports of a particular bridge may be associated with any number of groups within the bridge by using a virtual local area network (VLAN) arrangement that virtually associates the port with a particular VLAN designation. More specifically, Ross discloses a bridge or hub that associates VLAN designations with at least one local port and further associates those VLAN designations with messages transmitted from any of the ports to which the VLAN designation has been assigned.
The VLAN designation for each local port is stored in a memory portion of the hub such that every time a message is received by the hub on a local port the VLAN designation of that port is associated with the message. Association is accomplished by a flow processing element which looks up the VLAN designation in the memory portion based on the local port where the message originated. In addition to the '402 patent, the Institute of Electrical and Electronics Engineers (IEEE) has issued a standard for Virtual Bridged Local Area Networks, identified as the IEEE 802.1Q specification standard.
In many cases, it may be desirable to interconnect a plurality of these bridges or hubs in order to extend the VLAN associations of ports in the network. Ross, in fact, states that an objective of his VLAN arrangement is to allow all ports and entities of the network having the same VLAN designation to exchange messages by associating a VLAN designation with each message. Thus, those entities having the same VLAN designation function as if they are all part of the same LAN. VLAN-configured bridges and hubs are specifically configured to prevent message exchanges between parts of the network having different VLAN designations in order to preserve the boundaries of each VLAN segment. Intermediate network devices operating above L2 , such as routers, can relay messages between different VLAN segments. The IEEE 802.1Q specification standard further calls for the addition of a VLAN Identifier (VID) field to the header of network messages. The VID field may be loaded with a numeric value (0-4095) corresponding to the message's VLAN designation. For administrative convenience, each VLAN designation is often associated with a different color, such as red, blue, green, etc.
FIG. 1 is a partial block diagram of a data link (e.g., Ethernet) frame 100 that complies with the IEEE 802.1Q specification standard. Frame 100 includes a header 102, a data field 104 and a frame check sequence (FCS) field 106. Header 102 includes a MAC destination address (DA) field 108, a MAC source address (SA) field 110 and a length/type field 111, which specifies the size of the data field 104. According to the 802.1Q specification standard, header 102 further includes a tag header 112 which is located immediately following the DA and SA fields 108, 110.
The tag header 112 includes a Tag Protocol Identifier (TPID) field 114, which identifies the frame as an IEEE 802.1Q type frame, and a tag control information field 116 which, in turn, comprises a plurality of sub-fields, including, a user_Priority field 118, a Canonical Format Indicator (CFI) field 120 and a Virtual Local Area Network Identifier (VID) field 122. The CFI field indicates whether the bit order of the VID field 122 is canonical or non-canonical. The VID field 122 is used to specify the frame's Virtual Local Area Network (VLAN) designation.
User Priority
The user_priority field 118 permits a network entity to select a desired priority that is to be applied to the frame 100. In particular, an IEEE appendix, referred to as the 802.1p specification standard, defines eight possible values of user priority (0-7), each of which is associated with a specific traffic type. The proposed user priority values and their corresponding traffic types, as specified in the 802.1p specification standard, are as follows.
UserPriorityValueTraffic TypeDescription1Backgroundbulk transfers2Spare/Reservedn/a0Best Effortcurrent LAN traffic3Excellent Effortbest effort type of services(e.g., for an organization'smost important customers)4Controlled Loadimportant business applications5Video (<100 millisecondsminimum jitterlatency and jitter)6Voice (<10 millisecondsone-way transmission throughlatency and jitter)the LAN7Network Controlcharacterized by a “must getthere” requirement to maintainand support the networkinfrastructure
An intermediate network device may have a plurality of transmission queues per port and, pursuant to the 802.1p specification standard, may assign frames to different queues of a destination port on the basis of the frame's user priority value. For example, frames with a user priority of “0” are placed in the “0” level queue (e.g., non-expedited traffic), whereas frames with a user priority of “3” are placed in the level “3” queue. Furthermore, frames stored in a higher level queue (e.g., level 3/excellent effort) are preferably forwarded before frames stored in a lower level queue (e.g., level 1/background). This is commonly referred to as Priority Queuing. Thus, by setting the contents of the user_priority field 118 to a particular value, a network entity may affect the speed with which the frame traverses the network.
If a particular intermediate network device has less than eight queues per port, several of the IEEE traffic types may be combined. For example, if only three queues are present, then queue 1 may accommodate best effort, excellent effort and background traffic types, queue 2 may accommodate controlled load and video traffic types and queue 3 may accommodate voice and network control traffic types. The IEEE 802.1p specification standard also recognizes that intermediate network devices may regenerate the user priority value of a received frame. That is, an intermediate network device may forward the frame with a different user priority value (still within the range of 0-7) than the one it had when the frame was received. Nevertheless, the standard recommends that the user priority value be left un-changed.
Metropolitan Area Networks (MANs)
In many instances, several LANs and/or end stations may be interconnected by point-to-point links, microwave transceivers, satellite hook-ups, etc. to form a metropolitan area network (MAN) that may span several city blocks, an entire city and/or an entire metropolitan area, such as the San Francisco Bay Area. The MAN typically interconnects multiple LANs and/or end stations located at individual campuses and/or buildings that are physically remote from each other, but that are still within the metropolitan area. Conventional MANs typically rely on network equipment employing Asynchronous Transfer Mode (ATM) running over the existing Public Switched Telephone Network's (PSTN's) Synchronous Optical Network (SONET). As most LANs utilize the Ethernet standard, network messages or frames created at one LAN must be converted from Ethernet format into ATM cells for transmission over the SONET links. The ATM cells must then be converted back into Ethernet format for delivery to the destination LAN or end station. The need to convert each network message from Ethernet to ATM and back again requires the MAN to include expensive networking equipment. The MAN Provider also has to lease or otherwise obtain access to the SONET links. As a result, MANs can be expensive to build and operate.
In addition, it would be desirable to connect many different networks to a MAN. At least some of these different networks, however, may nonetheless be using the exact same VLAN designations to identify their traffic. Once such traffic enters the MAN, it can be difficult to ascertain from which particular network the traffic originated. This could result in forwarding errors by the MAN.